The invention relates to a steering booster configuration for motor vehicles according to the preamble of claim 1.
Known servo steering systems are operated, for example hydraulically supported, thereby that on the steering shaft the control torque generated by the driver is measured and therewith via a hydraulic system a piston is driven which acts upon the transverse toothed rack of the steering system, which is a portion of a thrust rod and thus generates a force augmentation depending on the deflection of the steering wheel. Such electrohydraulic systems have the disadvantage that at high vehicle speeds, where per se the least servo force augmentation is required, the greatest hydraulic power is obtained and during standstill of the motor vehicle the force augmentation steeply decreases, where the greatest steering forces would be required.
Attempts have therefore been made to realize steering booster configurations which drive the toothed rack directly with an electromotor. Such systems save space and energy since the electromotor is only activated when needed. With electromotor-driven steering booster configurations it would be possible to provide the auxiliary force supply at the correct point in time, for example correspondingly more strongly when the motor vehicle is not moving.
U.S. Pat. No. 5,711,396 for example discloses an electromotor-driven servo drive, which acts directly onto the toothed rack of the steering system. In order to be able to generate sufficiently high forces, a correspondingly robust electromotor is provided which requires corresponding strong stepping-down in order to generate sufficiently high controlling forces. In the prior cited patent the electromotor was disposed laterally to the toothed rack or thrust rod, which drives a ball spindle gear which is disposed coaxially with the toothed rack axis and serves as a reduction gear. Of disadvantage in this configuration is that the motor and the gear disposition are of a complicated structure and occupy a relatively large installation space. In addition, the efficiency is not particularly high. To save installation space, attempts have also been made to dispose recirculating ball gears together with the electromotor coaxially on the toothed rack axis, such as has become known for example from GB 2 284 790. A further coaxial configuration was disclosed in EP 0 101 579. The configurations known until today have the disadvantage that their structure is relatively complicated, in particular in the proximity of the bearing of the ball nut and of the motor rotor. Due to the limited efficiencies of the electrically operated servo steering configuration, the application field is also limited since these also demand high electric feed power of the electromotor. The available electric power of the onboard network of a motor vehicle is limited. In addition, the previous steering boosters are insufficiently compact, which also limits utilization in small motor vehicles or makes them not economical.
The task of the present invention resides in eliminating the disadvantages of the above cited prior art. The task resides in particular in realizing an electric steering booster for a steering configuration, which is structured extremely compactly and which permits attaining rapid response behavior with good dynamics with responsive steering behavior for the driver, which operates at high efficiency and which can generate large controlling forces acting directly onto the thrust rod or toothed rack and which can be produced economically. The configuration, moreover, is to be highly compact and is to have the capacity of being disposed tubularly coaxially with respect to the axis of the thrust rod.
The task is solved according to the invention by the configuration according to the characteristics of claim 1. The dependent claims define further advantageous embodiments.
The task is solved according to the invention thereby that an electromotor disposed coaxially to the thrust rod axis drives with its rotor a rotation-translation converter. This comprises a ball-threaded nut, which, in turn engages a threaded coupling disposed in a subregion of the thrust rod. The rotatable nut, or the rotor, are rotatably supported with only one bearing and this is stationarily braced against the motor vehicle chassis, such that upon torquing of the nut it causes an axial displacement of the thrust rod, wherewith a corresponding force acts onto the thrust rod. The nut and the thrust rod act like a gear for the force transmission. The advantage of this configuration comprises that the rotor of the motor is coupled directly with the ball-threaded nut of the rotation-translation converter and only one bearing is required.
As the rotation-translation force converter the already known recirculating ball spindle gears or recirculating-ball worm gears are especially suitable. In such configurations the balls, which roll along between the nut and the threaded turn of the threaded rod, for example in an endless loop, are guided back again to their original position. An especially suitable configuration of a recirculating-ball worm gear is available for example from the company Amsler and Co AG in CH-8245 Feuerthalen with the type designation xe2x80x9cStarxe2x80x9d and the name xe2x80x98Kugelgewindetriebxe2x80x99 (ball screw). With this model an especially compact, precise converter configuration with high efficiency can be attained. The utilization of a special, electronically commutating motor with bell-like rotor, moreover, permits the use of a single bearing for the bearing support of the ball-threaded nut together with the bell-like rotor. The electromotor disposed coaxially with the toothed rack or thrust rod axis is developed such that the stator with its winding coaxially encompasses the thrust rod axis and the rotor, preferably tubularly, encompasses rotatably supported the stator thereover. The rotor carries permanent magnets of the type rare earths and is developed as a bell-form rotor, which is supported on one side together with the ball-threaded nut. With such a configuration high degrees of efficiency can be attained with compact implementation.
The force transmission ratio of the rotation-translation converter and of the motor is laid out such that high controlling forces can be generated, preferably up to more than 9 kN with rapid reaction times, which is of particular importance for the steering safety. The motor is frequency-controlled according to the desired requirements via an electronic power stage, wherein the driving frequency lies preferably above the range of the human audibility limit. The power stage is driven by a control unit, which processes as instantaneous value signal the torque measured on the steering shaft, which is generated during the steering with respect to the toothed rack, for this purpose for example a torsion rod is disposed between the steering gearing and the steering shaft and its deformation under the effect of force is measured. The control can with advantage be provided with a corresponding characteristic or transfer function in order to be able to generate the desired control characteristics at different operating behavior. For example, the speed-dependent and weight-dependent control force behavior can be taken into consideration and be compensated. Furthermore, with advantage the resetting of the front wheels upon releasing the steering wheel can be supported and/or irregularities of the steering joint, for example due to a universal joint error, can be compensated and/or rotational vibrations of the steering system can be actively damped. The electronically controlled motor permits, moreover, to equalize certain negative properties of the motor, such as for example residual ripple behavior or also fabrication tolerances, thereby that, for example, the motor power is compensatingly modulated either through fixed presetting or by measuring the effect via sensors and integration into a programmed control system. The control is advantageously developed as programmable control, for example as a microprocessor system, which can be defined through software or also through fixedly integrated, stored programs. With the motor converter configurations according to the invention good overall degrees of efficiency with compact structure and with high controlling forces can be achieved without overloading of the onboard network.